2 marks q&a


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2 marks q&a

  1. 1. 2MARKS QUESTION & ANSWERS UNIT-1 DIGITAL COMMUNICATION SYSTEM 1. Define modulation? Modulation is a process by which some characteristics of high frequency carrier signal is varied in accordance with the instantaneous value of the modulating signal. 2. What is correlative coding? Correlative level coding is used to transmit a baseband signal with the signaling rate of 2Bo over the channel of bandwidth Bo. This is made physically possible by allowing ISI in the transmitted in controlled manner. This ISI is known to receiver. The correlative coding is implemented by duobinary signalling and modified duobinary signalling. 3. State sampling theorem. · If a finite –energy signal g(t) contains no frequencies higher than W hertz ,it is completely determined by specifying its co=ordinates at a sequence of points spaced 1/2W seconds apart. · If a finite energy signal g(t) contains no frequencies higher than W hertz, it may be completely recovered from its co=ordinates at a sequence of points. 4. What are the degrees of modulation? Under modulation. m<1 Critical modulation m=1 Over modulation m>1 5. What is aliasing? The phenomenon of a high-frequency in the spectrum of the original signal g(t) seemingly taking on the identity of a lower frequency in the spectrum of the sampled signal g(t) is called aliasing or foldover. 6. Why do you need adaptive equalization in a switched telephone network. In switched telephone network the distortion depends upon 1) Transmission characteristics of individual links. 2) Number of links in connection. Hence fixed pair of transmit and receive filters will not serve the equalization problem. The transmission characteristics keep on changing. Therefore adaptive equalization is used. 7.What are the necessity of adaptive equalization?
  2. 2. Ans. Most of the channels are made up of individual links in switched telephone network, the distortion induced depends upon 1) transmission characteristics of individual links 2) number of links in connection 8. Define the principle of adaptive equalization? Ans. The filters adapt themselves to the dispersive effects of the channel that is the cofficients of the filters are changed contineously according to the received data. The filter cofficients are changed in such a way that the distortion in the data is reduced 9. Define duobinary encoding? Ans. Duobinary encoding reduces the maximum frequency of the base band signal the “word duo” means to the double transmission capacity of the binary system 10. Write a note on correlative level coding? Correlative level coding allows the signal scaling rate of 2Bo in the channel of bandwidth Bo this is made physically possible by allowing ISI in the transmitted signal in controlled manner this ISI IS KNOWN TO THE RECEIVER 11. Define the term ISI? Ans. The presence of outputs due to other bits interference with the output of required bit . this effect is called inter symbol interference (ISI) 12. Write the performance of data transmission system using eye pattern technique? Ans. The width of the eye opening defines the interval over which the received wave can can be sampled without error from inter symbol interference . The sensitivity of the system to timing error is determined by the rate of closure of the eye as the sampling time is varied. 13. What is the necessity of equalization? Ans. When the signal is passed through the channel distortion is introduced in terms of 1) amplitude 2) delay this distortion creates problem of ISI. The detection of the signal also become difficult this distraction can be compensated with the help of equalizer. 14. What is meant by prediction error? The difference between the actual sample of the process at the time of interest and the predictor output is called a prediction error. 15. Define delta modulation. Delta modulation is the one-bit version of differential pulse code modulation.
  3. 3. 16. Define adaptive delta modulation. The performance of a delta modulator can be improved significantly by making the step size of the modulator assume a time- varying form. In particular, during a steep segment of the input signal the step size is increased. Conversely, when the input signal is varying slowly, the step is reduced , In this way, the step size is adapting to the level of the signal. The resulting method is called adaptive delta modulation (ADM). UNIT -2 : BASEBAND FORMATTING TECHNIQUES 1. Mention the need of optimum transmitting and receiving filter in baseband data transmission. When binary data is transmitted over the baseband channel, noise interfaces with it. Because of this noise interference, errors are introduced in signal detection. Optimum filter performs two functions while receiving the noisy signal: 1) Optimum filter integrates the signal during the bit interval and checks the output at the time instant where signal to noise ratio is maximum 2) Transfer function of the optimum filter is selected so as to maximise signal to noise ratio. 3) Optimum filter minimizes the probability of error. 2. Define ASK. In ASK, carrier is switched on when binary 1 is to be transmitted and it is switched off when binary D is to be transmitted ASK is also called on-off keying. 3. What is meant by DPSK? In DPSK, the input sequence is modified. Let input sequence be d(t) and output sequence be b(t). Sequence b(t) changes level at the beginning of each interval in which d(t)=1 and it does not changes level when d(t)=0. When b(t) changes level, phase of the carrier is changed. And as stated above, b(t) changes t=its level only when d(t) =1. This means phase of the carrier is changed only if d(t)=1. Hence the technique is called Differential PSK. 4. Explain coherent detection? In coherent detection, the local carrier generated at the receiver is phase locked with the carrier at the transmitter. The detection is done by correlating received noisy signal and locally generated carrier. The coherent detection is a synchronous detection.
  4. 4. 5. What is the difference between PSK and FSK? In PSK, phase of the carrier is switched according to input bit sequence. In FSKfrequency of the carrier is switched according to input bit sequence. FSK needs double of the bandwidth of PSK. 6. What is meant by coherent ASK? In coherent ASK, correlation receiver is used to detect the signal. Locally generated carrier is correlated with incoming ASK signal. The locally generated carrier is in exact phase with the transmitted carrier. Coherent ASK is also called as synchronous ASK. 7. What is the major advantage of coherent PSK over coherent ASK? ASK is on-off signalling, where as the modulated carrier is continuously transmitted in PSK. Hence peak power requirement is more ASK, whereas it is reduced in case of PSK. 8. How are the predictor coefficients determined? For the adaptation of the predictor coefficients the least mean square (LMS) algorithm is used. 9. Define adaptive subband coding? It is a frequency domain coder, in which the speech signal is divided in to number of subbands and each one is coded separately. It uses non masking phenomenon in perception for a better speech quality. The noise shaping is done by the adaptive bit assignment. 10. What are formant frequencies? In the context of speech production the formant frequencies are the resonant frequencies of the vocal tract tube. The formants depend on the shape and dimensions of the vocal tract. 32. Define Adaptive filter? It is a nonlinear estimator that provides an estimate of some desired response without requiring knowledge of correlation functions, where the filter coefficients are data dependent. A popular filtering algorithm is the LMS algorithm. 11. Define data Signalling Rate. Data signalling rate is defined as the rate measured in terms bits per second(b/s) at which data are transmitted. Data signaling rate Rb=I/Tb Where Tb=bit duration. 12. Define modulation rate. It is defined as the rate at which signal level is changed depending On the nature of the format used to represent the digital data.It is measured in Bauds or symbols per second.
  5. 5. 13. State NRZ unipolar format In this format binary 0 is represent by no pulse and binary 1 is Represented by the positive pulse. 36. State NRZ polar format. Binary 1 is represented by a positive pulse and binary 0 is represented by a Negative pulse. 14. State NRZ bipolar format. Binary 0 is represented by no pulse and binary one is represented by the alternative positive and negative pulse. 15. State Manchester format. Binary 0 Æ The first half bit duration negative pulse and the second half Bit duration positive pulse. Binary 1Æ first half bit duration positive pulse and the second half Bit duration negative pulse. UNIT – 3: BASEBAND CODING TECHNIQUES 1. Properties of matched filter. The signal to noise ratio of the matched filter depends only upon the the ratio of the signal energy to the psd of white noise at the filter input 1) The output signal of a matched filter is proportional to a shifted version of the autocorrelation function of the input signal to which the filter is matched. 2. Why do we go for Gram-Schmidt Orthogonalization procedure? Consider a message signal m. The task of transforming an incoming message mi=1,2,…..M, into a modulated wave si(t) may be divided into separate discrete time & continuous time operations. The justification for this separation lies in the Gram-Schmidt orthogonalization procedure which permits the representation of any set of M energy signals, {si(t)}, as linear combinations of N orthonormal basis functions, where N £M . 3. What is matched filter receiver ? A filter whose impulse response is a time reversed & delayed version of some signal Æ j (t) then it is said to be matched to Æj (t) correspondingly, the optimum receiver based on the detector is referred to as the matched filter receiver. 4. What is maximum likelihood detector. Maximum likelihood detector computes the metric for each transmitted message compares them and then decides in favor of maximum. The device for implementing the decision rule
  6. 6. i.e; set ^m = mi if In [ fx(x/mk)] is maximum for k=i is called maximum –likelihood detector and the decision rule is called maximum likelihood. 5. Define antipodal signals. A pair of sinusoidal signals that differ only in a phase shift of 180 degrees are referred to as antipodal signals. 6. Explain how QPSK differs from PSK in term of transmission bandwidth and bit information it carries? For a given bit rate 1/Tb, a QPSK wave requires half the transmission bandwidth of the corresponding binary PSK wave. Equivalently for a given transmission bandwidth, a QPSK wave carries twice as many bits of information as the corresponding binary PSK wave 7. Give the equation for average probability of symbol error for coherent binary PSK. Average probability of signal error, Pe = 1 / 2 erfcÖ Eb / No 8. Explain the model of bandpass digital data transmission system? The bandpass digital data transmission system consists of source, encoder and modulator in the transmitter. Similarly receiver, decoder and destination form the transmitter. 9. What is baseband signal receiver? A baseband signal receiver increases the signal to noise ratio at the instant of sampling. This reduces the probability of error. The baseband signal receiver is also called optimum receiver. 10. What is matched filter? The matched filter is a baseband signal receiver, which works in presence of white Gaussian noise. The impulse response of the matched response of the matched filter is matched to the shape pf the input signal. 11. What is the value of maximum signal to noise ratio of the matched filter? When it becomes maximum? Maximum signal to noise ratio is the ratio of energy to psd of white noise. i.e., ρmax = E/ (N0/2) This maximum value occurs at the end of bit duration i.e. Tb. 12. What is correlator ? Correlator is the coherent receiver. It correlates the received noisy signal f(t) with the locally generated replica of the unknown signal x(t). It‟s output is denoted as r(t).
  7. 7. 13. On what factor, the error probability of matched filter depends. Error probability is given as Pe = 1/2erfc√E/No This equation shows that error probability depends only on energy but not on shape of the signal. 14. Bring out the difference between coherent & non coherent binary modulation scheme. a. Coherent detection: In this method the local carrier generated at the receiver is phase locked with the carrier at the transmitter. Hence it is called synchronous detection b. Non coherent detection: In this method, the receiver carrier need not be phase locked with transmitter carrier. Hence it is called envelope detection. 15. Write the expression for bit error rate for coherent binary FSK. Bit error rate for coherent binary FSK is given as, Pe = 1/2erfc√0.6E/No 16. Highlight the major difference between a QPSK & MSK signal. MSK signal have continuous phase in all the cases, where as QPSK has phase shift of π/2 or π. 17. What is the error probability of MSK & DPSK? Error probability of MSK: Pe = 1/2erfc√E/No Error probability of DPSK: Pe = 1/2e-Eb/No 18. In minimum shift keying what is the relation between the signal frequencies & bit rate. Let the bit rate be fb and the frequency of carrier be f0. The higher and lower MSK signal frequencies are given as, fH = f0 + fb/4 fL = f0 - fb/4 UNIT- 4: BASEBAND RECEPTION TECHNIQUES 1.What is hamming distance? The hamming distance between two code vectors is equal to the number of elements in which they differ. For example, let the two code words be, X = (101) and Y= (110) These two code words differ in second and third bits. Therefore the hamming distance between X and Y is two. 2. Define code efficiency. The code efficiency is the ratio of message bits in a block to the transmitted bits for that block by the encoder i.e., Code efficiency= (k/n)
  8. 8. k=message bits n=transmitted bits. 3. What is meant by systematic and non-systematic codes? In a Systematic block code, message bits appear first and then check bits. In the non-systematic code, message and check bits cannot be identified in the code vector. 4. What is meant by linear code? A code is linear if modulo-2 sum of any two code vectors produces another code vector. This means any code vector can be expressed as linear combination of other code vectors. 5. What are the error detection and correction capabilities of hamming codes? The minimum distance (dmin) of hamming codes is „3‟. Hence it can be used to detect double errors or correct single errors. Hamming codes are basically linear block codes with dmin =3. 6. What is meant by cyclic codes? Cyclic codes are the subclasses of linear block codes. They have the property that a cyclic shift of one codeword produces another code word. 7. How syndrome is calculated in Hamming codes and cyclic codes? In hamming codes the syndrome is calculated as, S=YHT Here Y is the received and HT is the transpose of parity check matrix. 8. What is BCH code? BCH codes are most extensive and powerful error correcting cyclic codes. The decoding of BCH codes is comparatively simpler. For any positive integer „m‟ and „t‟ (where t<2 m-1)there exists a BCH code with following parameters: Block length: n= 2m-1 Number of parity check bits : n-k<=mt Minimum distance: dmin>=2t+1 9. What is RS code? These are non binary BCH codes. The encoder for RS code operates on multiple bits simultaneously. The (n, k) RS code takes the groups of m- bit symbols of incoming binary data stream. It takes such „k‟ number of symbols in one block. Then the encoder acts (n – k) redundant symbols to form the code word of „n‟ symbols RS code has: Block Length : n=2m-1 symbols Message size: K symbols
  9. 9. Parity check size: n-k= 2t symbols Minimum distance: dmin=2t+a symbols 10. What is difference between block codes and convolutional codes? Block codes takes‟k‟ number of bits simultaneously form „n‟-bit code vector. This code vector is also called block. Convolutional code takes one message bits at a time and generates two or more encoded bits. Thus convolutional codes generate a string of encoded bits for input message string. 11. Define constraint length in convolutional code? Constraint length is the number of shift over which the single message bit can influence the encoder output. It is expressed in terms of message bits. 12. Define free distance and coding gain. Free distance is the minimum distance between code vectors. It is also equal to minimum weight of the code vectors. Coding gain is used as a basis of comparison for different coding methods. To achieve the same bit error rate the coding gain is defined as, A= (Eb/No) encoded (Eb/No) coded For convolutional coding, the coding gain is given as, A = rdf /2 Here „r‟ is the code rate And „df is the free distance. 13. What is convolution code? Fixed number of input bits is stored in the shift register & they are combined with the help of mod 2 adders. This operation is equivalent to binary convolution coding. 14. What is meant by syndrome of linear block code? The non zero output of the produce YHT is called syndrome & it is used to detect errors in y. Syndrome is denoted by S & given as, S=YHT. UNIT- 5: BANDPASS SIGNAL TRANSMISSION AND RECEPTION 1. Why synchronization is required ? The signals from various sources are transmitted on the single channel by multiplexing. This requires synchronization between transmitter and receiver. Special synchronization bits are added in the transmitted signal for the purpose. Synchronization is also required for detectors to recover the digital data properly from the modulated signal.
  10. 10. 2. What is pseudo noise sequence ? Ans. Pseudo noise sequence is a noise like high frequency signal. The sequence is not completely random, but it is generated by well defined logic. Hence it is called„pseudo‟ noise sequence. Pseudo noise sequences are used in spread spectrum communication for spreading message signals. 3. What is direct sequence spread spectrum modulation Ans. In direct sequence spread spectrum modulation, the pseudo-noise sequence is directly modulated with data sequence. Thus pseudo-noise sequence acts as high frequency carrier and data sequence acts as low frequency modulating signal. The pseudo-noise sequence and data sequence are applied to a product modulator. The output of product modulator can be used directly or it can further generate BPSK signal for long distance communication. 4. What is frequency hap spread spectrum modulation? Ans. In frequency hop spread spectrum, the data is transmitted in different frequency slots. These frequency slots are selected with the help of pseudo-noise sequence. Selection of frequency slots is called frequency hopping. The bandwidth of frequency hop spread spectrum is very much large compared to direct sequences spread spectrum. Frequency hop spread spectrum is of two types: I) slow frequency hopping and ii) fast frequency hopping. 5. Define code efficiency. It is the ratio of message bits in a block to the transmitted bits for that block by the encoder. 6. What is jamming margin ? Ans. Jamming margin is the ratio of average power of interference (J) to average power of data signal (Ps) 7. State four applications of spread spectrum. I) Spread spectrum has the ability to resist the effect of intentional jamming ii) Spread spectrum is used in mobile communications. This is because the spread spectrum signal has the ability to resist the effects of multipath fading. 8. When is the PN sequence called as maximal length sequence? Ans. When the PN sequence has the length of 2m - 1, it is called maximal length sequence.